The present invention relates to a method and apparatus for casting with an aluminum alloy or the like an engine block for an automobile or the like having a plurality of cylinders arranged in, e.g., tandem with each other.
In an engine block, a water jacket serving as a space for flowing cooling water is formed in an peripheral portion of a cylinder bore at a position slightly spaced from the cylinder bore.
Open and closed deck engine blocks are prepared in accordance with different water jacket formation techniques. In the open deck engine block, the upper surface of a water jacket is entirely open at a head cover contact surface of the upper surface of the engine block. In the closed deck engine block, although the interior of a water jacket is continuous along the entire periphery, the upper surface of the water jacket is partially open at a plurality of locations at the head cover contact surface. That is, bridge portions which connects opposite sides of the water jacket are respectively formed at the plurality of locations of the upper surface of the water jacket.
Open deck engine blocks have been conventionally employed. In this case, a normal metal core can be used to form a space serving as the water jacket. No problem is posed to form the water jacket, and formation can be facilitated. In the open deck engine block, since the entire periphery of the upper surface of the water jacket is open, problems on the strength and deformation of the engine block itself are posed. As a result, in order to solve these problems, the wall thickness of the engine block must be increased, the overall weight of the engine must be increased, and fuel consumption is undesirably increased.
In recent years, the closed deck engine block has received a great deal attention. Some manufacturers attempt to manufacture closed deck engine blocks.
The closed deck engine block is said to be excellent in strength and against deformation as compared with the open deck engine block. Since the plurality of bridge portions as closed portions are formed at upper surface portions of the water jacket, a metal core cannot be used, unlike in a conventional technique. A destructive core is used in place of this metal core because the destructive core can be destructed and removed upon casing of the engine block. Although a sand core, a salt core, and the like may be used as destructive cores, the sand core is most popular.
A sand core cannot be simply used to form a water jacket serving as a space in an engine jacket.
The engine block particularly requires a high strength and durability, and any cavity must not be formed therein. As has been attempted in these days, when an engine block is made of a light alloy such as an aluminum or magnesium alloy in place of cast iron in favor of a lightweight structure, a fine product without any cavity is required. For this reason, a casting method free from formation of cavities is required, and high-pressure casting is also required. Formation of molten metal solidified pieces called burrs on separation and sliding surfaces of the molds during casting must be minimized. Even if burrs are formed, they must be easily removed so that they do not interfere with the next casting cycle.
On the other hand, when a molten metal is to be injected into the molds at a high pressure, the sand core must not be deformed, destructed, or cracked. After casting, the sand core must be destructed, and all the sand must be easily and properly removed from the cast engine block. For this purpose, a special-purpose sand core must be employed. Special care must be taken for casting, and special implementations must be provided in a casting apparatus. When a sand core is deformed during casting, a hole formed in a water jacket of an engine block is deformed, a thin-walled portion is formed in a thick wall of the engine block to degrade the strength and durability and cause cooling water leakage. In addition, when the sand core is destructed or cracked during casting, the product itself becomes defective. After casting, if all the sand cannot be removed, and sand is removed from the engine block during its use, the sand flows through a cooling water circuit, thereby adversely affecting operations of a cooling water pump and its valve and causing, in the worst case, operation failures.
In an existing engine block, a cast iron cylinder liner is mounted in the peripheral surface of the cylinder bore. In the near future, a cylinder liner may not be used due to a material improvement.
When an engine block is cast using a cylinder liner, before and during casting, a special implementation must be provided to mount a cylinder liner in part of a mold. More specifically, when a cylinder liner is mounted in and held by part of the mold, the cylinder liner must be smoothly mounted in the mold, as a matter of course. Cracking of the sand core and its partial damage, caused by a shock or the like during mounting of the cylinder liner in the mold must be prevented.
A conventional apparatus for casting an engine block of this type is disclosed in Japanese Patent Laid-Open No. 61-180661. This casting apparatus comprises a lower mold fixed on a stationary platen. An upper mold which is supported on a movable platen and lifted together with the movable platen by a mold clamping cylinder is arranged above the lower mold. A plurality of slide molds which are divided in the circumferential direction and are opened/closed by opening/closing cylinders upon radially horizontal movement are supported on the lower mold side. A plurality of blocks having an almost semicircular section and arranged in tandem with each other in the longitudinal direction of the lower mold extend from the lower mold at a contact portion of the closed slide molds. Arcuated portions for forming a cavity corresponding to a crank case, together with the blocks during mold closing, are formed in the lower halves of the slide molds. A destructive sand core obtained by connecting four cylinders in tandem with each other extends upward from and supported on the upper ends of the blocks. Four columnar members on which cylinder liners are fitted are suspended from the upper mold in correspondence with the cylindrical portions of the sand core. The stationary sleeve formed on the lower mold and communicating with an injection sleeve communicates with the cavity corresponding to the crank case and the cavities formed on both sides of the sand core during clamping between the upper mold and the slide molds. A plurality of temporary setting pins for holding the sand core are formed on the lower mold so as to extend upward from pin holes of the blocks by means of springs and the like.
With the above arrangement, the sand core is placed on the temporary setting pins slightly extending from the blocks between the molds. In this state, the slide molds are closed to insert skirt portions as a plurality of projections formed on the outer side surface of the sand core into core holding holes formed inside the slide molds, thereby holding the sand core. Thereafter, the temporary setting pins are retracted into the blocks. The upper mold is moved downward by the mold clamping cylinder and is urged against the lower mold. The four columnar members suspended from the upper mold are moved downward together with the cylinder liners and inserted into the sand core cylindrical portions which support the skirt portions. Before the slide mold and the upper mold are clamped against each other, a molten metal is injected from an injection sleeve also serving as the stationary sleeve in which the molten metal is charged in advance. The cavity corresponding to the crank case which communicates with the injection sleeve and the cavities contacting both surfaces of the sand core are filled with the molten metal, and the molten metal is solidified. When the upper mold and the slide molds are opened, and the push pins mounted on the upper mold are extended, the product solidified in the cavity, i.e., the engine block is released from the molds. Thereafter, when the sand core in the engine block serving as a product is destructed, the sand core is removed in the form of small destructed pieces. A cooling water circulating jacket serving as a space can be formed.
In the conventional engine block casting apparatus described above, however, each slide mold is supported on the lower mold and is horizontally reciprocated while sliding along the upper surface of the lower mold located near a casting side on which a high-temperature molten metal is injected. The molten metal enters into a gap between the slide molds and the lower mold to tend to form burrs. These burrs cannot be easily removed, and the slide molds will not move, thus interrupting a casting operation. In addition, the molten metal inserted into the above gap leaks outside the molds to endanger workers. In addition, the amount of molten metal becomes short, thus degrading product quality.
If the burr is present between sliding surfaces, i.e., the lower surfaces of the slide molds and the upper surface of the lower mold, it is difficult to release the product from the molds. When the slide mold is open and when the burr is left on the upper surface of the lower mold or a burr is dropped from the above, the bur cannot be perfectly eliminated to the outside of the casting apparatus even by air blowing due to the presence of the slide molds. In addition, since a fresh high-temperature molten metal injected from the injection sleeve is brought into direct contact with the lower surfaces of the slide molds, a heat check occurs in each slide mold, and the slide molds will not open. In addition, since the lower surfaces of the slide molds are always in contact with the upper surface of the lower mold, the lower surface of each slide mold cannot be sprayed, or externally cooled or cleaned, resulting in inconvenience.
In the conventional casting apparatus, since the skirt portions are used to hold the scan core in the slide molds, a molded body has holes corresponding to the skirt portions. Therefore, these holes must be embedded with an aluminum alloy or the like after the sand core is removed.
When the engine block is to be cast using the sand core, as described above, the sand core must have a sufficiently high strength so as to prevent its destruction and deformation during casting of the molten metal at a high pressure. At the same time, after casting, when the product is to be released from the molds and the sand core is to be removed, all the sand must be easily and properly removed. For this purpose, special binders may be mixed in the sand, or a special coating may be formed on the surface of the sand core.
During high-temperature casting, gases may be produced from these binders or the like. In addition, air and a gas as of a mold release agent are also present in the mold cavity. If these gases are not sufficiently removed outside the molds at the time of casting, a cavity may be formed in a product, and the sand core is destructed or damaged during casting using the molten metal. When a small amount of gas is left in the mold cavity and is not sufficiently discharged during casting and is moved to a corner of the mold cavity, this gas is heat-insulatively compressed by the behavior of the molten metal. The mold portion corresponding to the compressed gas is set at an extremely high temperature. For example, although an aluminum alloy subjected to casting has a melting temperature of about 700.degree. C., the gas has a high temperature of 1,000.degree. C. or more by heat-insulative compression. By this high temperature, a binder in a sand core is thermally decomposed to produce a gas. At the same time, a degree of bond of the sand particles is decreased to cause the drawbacks described above.
When casting using a special sand core, as in this engine block, must be performed at a high pressure, discharge of gases is one of the most important problems to be solved.
When a large, complicated cast product requiring high quality and a high strength, as in an engine block, is to be cast, only a fresh, high-quality molten metal must always be used. Injection of a solidified component of the molten metal must be minimized as much as possible. For this purpose, the molten metal must be quickly cast.
The present invention has been made in consideration of this point, too.